1. Field of the Invention
This invention relates generally to a semiconductor memory element, and, more particularly, to forming a double-doped polysilicon floating gate in a semiconductor memory element.
2. Description of the Related Art
Data storage devices in modern integrated circuits generally include a plurality of memory cells formed above a semiconductor substrate, such as silicon. For example, a semiconductor memory array may include 256K (256×1024) memory cells. Electrically conducting lines are also formed in the semiconductor substrate and coupled to the memory cells. Bits of data are stored in the memory cells, for example, by providing a voltage or a current to a plurality of bit lines and a plurality of orthogonal word lines that are electrically coupled to the memory cells. In one embodiment, the memory cells are formed from non-volatile components, such as a floating gate transistor. For example, floating gates are used to form flash memory cells, EEPROM memory cells, and the like.
A traditional flash memory cell 100 is shown in FIG. 1. The flash memory cell 100 includes a source 110 and a drain 115 formed in a substrate 120, which may be comprised of a variety of semiconductor materials. For example, the substrate 120 may be comprised of silicon, which may be doped with n-type or p-type dopants. During operation of the flash memory cell 100, a channel 125 will be established in the substrate 120 between the source and drain regions 110, 115. The flash memory cell 100 also includes a first insulating layer 130 that is often called a tunnel oxide layer, a floating gate 105, a second insulating layer 135 that is often called an inter-poly insulation layer, and a control gate 140. The insulating layers 130, 135 may be dielectric layers. In the interest of clarity, the techniques for forming the various gates and layers described above, which are well known to those of ordinary skill in the art and are not material to the present invention, will not be discussed herein.
To program the flash memory cell 100, a first voltage V1 is provided to the control gate 140. A second voltage V2, which is usually smaller than the first voltage V1, is provided to the drain 115, and third and fourth voltages V3, V4, which are generally smaller than the second voltage V2, are provided to the source 110 and the substrate 120, respectively. For example, a first voltage V1 of about 8-10 volts is provided to the control gate 140, a second voltage V2 of about 4-5 volts is provided to the drain 115, and the source 110 and substrate 120 are grounded.
The voltage of about 8 volts on the control gate 140 and the resulting voltage differential of about 4 volts between the source 110 and the drain 115 will cause a current of electrons to flow through the channel 125 from the source 110 to the drain 115. As electrons flow through the channel 125, the voltage differential of about 8 volts between the control gate 140 and the substrate 120 will cause a portion of the electrons to “tunnel” through the first insulating layer 130 to the floating gate 105 and become trapped or stored therein. The presence or absence of the collected electrons in the floating gate 105 may be detected in a reading operation well known to those of ordinary skill in the art. For example, the presence of collected electrons in the floating gate 105 may be determined to represent a logic-low state, whereas the absence of collected electrons in the floating gate 105 may be determined to represent a logic-high state, or vice versa.
To erase the flash memory cell 100 once it has been programmed, different voltage levels are generally applied to the flash memory cell 100. For example, a first voltage V1 of about negative 9 volts is provided to the control gate 140, a third voltage V3 of about +9 volts is provided to the source 110, and a fourth voltage V4 of about +9 volts is provided to the substrate 120. The resulting voltage differential of about 18 volts between the control gate 140 and the substrate 120 causes a portion of the electrons in the floating gate 105 to “tunnel” through the first insulating layer 130 to the substrate 120, thus discharging the floating gate 105. In one embodiment, a second voltage V2 may be allowed to float.
Although the flash memory cell 100 described above generally traps or stores electrons for a relatively long time compared to volatile memory cells such as DRAMs, and the like, a leakage current formed of electrons tunneling from the floating gate 105 to the substrate 120 may eventually discharge the flash memory cell 100. Furthermore, the effects of a leakage current generally increase as the size of non-volatile memory cells, such as the flash memory cell 100, decrease. Thus, future advances in semiconductor processing technology that tend to reduce the size of non-volatile memory cells will only exacerbate this problem.
The effects of the leakage current may be reduced by improving the insulating capabilities of the first and/or second insulating layers 130, 135 surrounding the floating gate 105. For example, the first and/or second insulating layers 130, 135 may be made thicker. However, such an approach adversely impacts the programming and erasing operations of the flash memory cell 100.
A method and structure are needed to reduce the effects of leakage current without unduly affecting the programming, reading, and/or erasure of the flash memory cell 100.